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A method for detector description exchange among ROOT GEANT4 and GEANT3

October 2010
Chinese Physics C 32(7):572

Authors:

Peking University

GEANT3 and GEANT4 are the detector simulation software programs that are widely used in most nuclear and particle physics experiments. ROOT is a program for framework, data analysis, online and offline software. Detector description is an important function in all these programs. Due to different detector construction methods and respective detector data representative, it is difficult to exchange the detector data among them. A new method based on GDML is developed to automatically convert the detector data among ROOT, GEANT4 and GEANT3. Any existing detector geometry in one program can be mapped to the geometry in the other two programs. In the software development of an experiment, different applications can share and reuse the same detector description. The application of this method in the PHENIX experiment upgrade and PHENIX Forward Vertex Silicon detector design is introduced.

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The PHENIX Forward Detector displayed with ROOT OpenGL graphic engine.

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Approximate boundary definition for a silicon sensor trapezoid.

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Chinese Physics C (HEP & NP) Vol. 32, No. 7, Jul., 2008

A method for detector description exchange among

ROOT GEANT4 and GEANT3*

YOU Zheng-Yun(cxm)1,2;1) LIANG Yu-Tie(ùc)1MAO Ya-Jun(kæ)1;2)

1 (School of Physics, Peking University, Beijing 100871, China)

2 (Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA)

Abstract GEANT3 and GEANT4 are the detector simulation software programs that are widely used in

most nuclear and particle physics experiments. ROOT is a program for framework, data analysis, online and

oﬄine software. Detector description is an important function in all these programs. Due to diﬀerent detector

construction methods and respective detector data representative, it is diﬃcult to exchange the detector data

among them. A new method based on GDML is developed to automatically convert the detector data among

ROOT, GEANT4 and GEANT3. Any existing detector geometry in one program can be mapped to the

geometry in the other two programs. In the software development of an experiment, diﬀerent applications can

share and reuse the same detector description. The application of this method in the PHENIX experiment

upgrade and PHENIX Forward Vertex Silicon detector design is introduced.

Key words detector description, GDML, GEANT3, GEANT4, ROOT

PACS 07.05.Tp, 07.05.Fb

1 Introduction

GEANT3[1], GEANT4[2] and ROOT[3] are soft-

ware programs that are widely used in nuclear

and particle physics experiments all over the world.

GEANT is a system of detector description and sim-

ulation tool that helps physicists to design and opti-

mize the detectors, develop and test the reconstruc-

tion and analysis programs, and interpret the exper-

iment data. The FORTRAN based GEANT3 was

developed by CERN from 1982 to 1994, and was

used in simulation programs of many experiments

like CDF, STAR and Pioneering High Energy Nuclear

Interaction eXperiment (PHENIX). The C++ based

GEANT4 with more powerful functions is developed

since 1999. ROOT is also developed by CERN to

meet the software requirement of the next generation

data analysis in experiments like the Large Hadron

Collider (LHC). With the development of object-

oriented technology, the C++ based ROOT is being

used for framework, online and oﬄine software in all

major high energy and nuclear physics laboratories

around the world, to monitor, store and analyze data.

Detector description is one of the most impor-

tant tasks in simulation tools like GEANT. In Monte

Carlo calculations of GEANT, a detector should be

described precisely and closely enough to reality, so

as to better simulate the response of the detectors to

the experimental conditions. The geometrical struc-

ture and properties of all the components of detectors

must be speciﬁed in detail. The speciﬁcations are of-

ten provided through function calls in the language

of the software libraries being used (FORTRAN for

GEANT3 and C++ for GEANT4). The detector

data exchange between GEANT3 and GEANT4 is

diﬃcult since they are based on diﬀerent computer

languages. Usually codes need to be rewritten while

upgrading from GEANT3 to GEANT4. Rewriting

source code is not only a large amount of work for

complex detectors, but also diﬃcult to keep consis-

tent in these two sets of description.

Detector description is also necessary in the re-

construction programs of online and oﬄine software.

To reconstruct the original tracks from the digital sig-

nal output, positions of the detecting units should be

known. Some more questions like “In which volume

is a given point?” and “Where is the boundary of a

volume?” should be answered. Mathematical calcu-

Received 21 September 2007

*Supported by NSFC (10675004, 10375002) and Doctoral Fund of Ministry of Education of China

1) E-mail: youzy@hep.pku.edu.cn

2) E-mail: maoyj@hep.pku.edu.cn

572 — 575

No. 7 YOU Zheng-Yun et alµA method for detector description exchange among ROOT GEANT4 and GEANT3 573

lations to resolve the coordinate transformation and

boundary search of the units are usually deﬁned by

the code developers. However, for complex detectors

with many levels of structure or detector units with

irregular shapes, this task becomes diﬃcult. A new

ROOT geometry package[4] with geometrical mode-

ling capabilities has been developed in the ROOT

framework since its 3.14 version. The architecture of

this modeler is a combination between a GEANT-like

containment scheme and a normal constructive solid

geometry (CSG) binary tree at the level of shapes.

The ROOT geometry package helps developers to

construct complex detectors and provides many func-

tions to support geometry service in reconstruction

software.

In the software development of an experiment, de-

tector description is used in diﬀerent applications like

simulation, reconstruction, visualization, alignment

and physics analysis. It is natural that only one set of

detector description should be shared and reused for

all applications so as to keep them consistent in an

experiment. One example is that the reconstruction

software processes both the simulation data and the

real data. The share of geometry between the simula-

tion and reconstruction software can reduce the sys-

tematic error that is brought by two sets of detector

description, so it is more reliable to compare the sim-

ulation data and the reconstruction data, especially

for complex detectors. However, in most experiments,

detector is described separately for each application.

Because of diﬀerent construction methods and diﬀer-

ent computer languages dependency, there is no eﬃ-

cient way to exchange the detector description among

these software programs.

2 Detector description exchange with

GDML

GDML (Geometry Description Markup Langu-

age)[5] is an XML (eXtensible Markup Language)[6]

based detector description language, which is devel-

oped by CERN and is particularly for GEANT4. All

necessary information to describe a detector is pro-

vided through a set of “tags” and “attributes” that

are parsed and passes through the code that provides

the Monte Carlo function calls in the proper formats.

The GDML description ﬁles are human readable and

plaintext which is editable by any ordinary text edi-

tor. GDML is platform (Windows or Linux or other

operating systems) independent and language (FOR-

TRAN or C++ or other languages) independent; it

provides an eﬃcient way to use a common source of

information for diﬀerent programs so long as the cor-

responding interfaces are developed.

A method based on GDML is developed to man-

age the detector description data and exchange them

among diﬀerent programs. We develop an interface

from GDML to ROOT geometry package and ex-

pand the functions of the GDML-GEANT4 interface

to support more complex shapes. The GDML ﬁles

can be read by GEANT4 to construct the detector

geometry for simulation, and at the same time, these

ﬁles can be read by ROOT to construct the detector

in ROOT for reconstruction, event display, analysis

and other ROOT-based applications. On the other

hand, the reverse conversion is also realized. For an

existing detector model that is already described with

GEANT4 or ROOT detector construction codes, the

GDML interfaces provide the functions to export the

detector data from GEANT4 or ROOT program to

GDML ﬁles, so as to reuse them in other programs.

GDML uses the XERCES-C++ parser and C++

code for producing the code syntax. The category

tags for parsing include:

(1) hnamespaceito set the ﬁelds where GDML is

used;

(2) hdeﬁneito set values like constant, position

and rotation;

(3) hmaterialsito specify all medium and materi-

als used in detector;

(4) hsolidsito describe more than 20 kinds of basic

shapes and complex Boolean shapes;

(5) hstructureito build the whole detector with

tree structure, by connecting a volume with its shape,

material, children volumes and mother volume.

Each time a tag is read, the GDML-ROOT in-

terface calls the corresponding deﬁnition or construc-

tion function in ROOT. When the ﬁle is completely

read in, the whole detector is constructed in appli-

cation as ROOT objects at the same time. The

GDML-GEANT4 interface works in a similar way.

The GDML detector description is program indepen-

dent; those who prepare the description ﬁles do not

need to know the functions in GEANT4 or ROOT.

So long as the GDML description ﬁle is provided,

the interface can automatically construct the detec-

tor in GEANT4 or ROOT’s respective construction

method. Automatic conversion not only reduces by

large amount the work of developers, but also guar-

antees the consistency of detector description in dif-

ferent applications.

For some large nuclear and particle physics experi-

ments that have already run several years, the simula-

tion programs were usually developed with GEANT3.

Sometimes there is a requirement to upgrade the

simulation program to GEANT4. A method to use

the existing GEANT3-based detector description for

the GEANT4-based or ROOT-based application is

also found. GEANT3 has the functions to export

the whole detector information, including structures,

574 Chinese Physics C (HEP & NP) Vol. 32

shapes, materials and volumes to a zebra-format ﬁle,

or to import the detector information from a zebra

format ﬁle. With the function “g2root”, which is

provided in the new ROOT geometry package, the ze-

bra ﬁle that contains the existing GEANT3 detector

information can be converted to the ROOT geome-

try representative. The GDML-ROOT interface has

the ability to transform the ROOT representative to

the GDML ﬁle and then the GDML ﬁle can be read

through the GDML-GEANT4 interface to construct

the detector in GEANT4, in which the detector de-

scription keeps exactly the same as that in GEANT3.

The upgrading from GEANT3 to GEANT4 is easily

achieved without rewriting the detector construction

codes in GEANT4.

The chain to exchange the detector description

data among GEANT3, GEANT4, ROOT and GDML

is shown in Fig. 1. All conversions are automatically

realized. The existing geometry in one program can

be mapped to representative in other programs and

can be reused in diﬀerent applications.

Fig. 1. Chain to exchange the detector descrip-

tion among GEANT3, GEANT4, ROOT and

GDML.

3 Applications in PHENIX experi-

ment

We ﬁrst successfully used the GDML-based de-

tector description in the software of BES0ex-

periment, for the GEANT4-based simulation and

ROOT-based reconstruction, event display and data

analysis[7]. Here we introduce how this method is

used in PHENIX[8], which is a high energy physics

experiment running on RHIC (Relativistic Heavy Ion

Collider) at Brookhaven National Laboratory. The

physics goals of PHENIX experiment include the

study of the attributes of nuclear matter under ex-

treme hot and dense conditions and spin structure of

nucleon. PHENIX project began in 1993 and started

collecting data in 2001.

3.1 Simulation upgrade from GEANT3 to

GEANT4

During the early stage of the PHENIX software

development in the 1990s, GEANT3 was the most

popular simulation tool and was used for PHENIX

simulation program. With GEANT4 fully developed

and being used in many particle experiments in recent

years, there is an intention to upgrade the PHENIX

simulation tool from GEANT3 to GEANT4. The

conversion from the GEANT3-based detector descrip-

tion to the GEANT4-based simulation program is

ﬁrst tested in the forward detectors of PHENIX.

In the forward direction, PHENIX has four sub-

detectors installed, namely, the Muon Tracker, the

Muon Identiﬁer, the Magnet and the Muon Pis-

ton Calorimeter, and two sub-detectors in upgrade,

namely, the Forward Vertex Silicon detector and the

Nose Cone Calorimeter. The current PHENIX sim-

ulation program has these six sub-detectors with de-

scription in the GEANT3 detector construction func-

tions. With the chain shown in Fig. 1, all information

of the forward detector is exported to a zebra format

ﬁle, and then it is converted to ROOT description

and exported in GDML format. Fig. 2 shows the

PHENIX forward detector converted from GEANT3

description. The detector is displayed with ROOT

OpenGL graphic engine.

Fig. 2. The PHENIX Forward Detector dis-

played with ROOT OpenGL graphic engine.

In the GEANT4-based new simulation program,

the detector construction is easily realized by import-

ing the GDML ﬁle with the GDML-GEANT4 inter-

face. Names of all deﬁnitions, solids, logical volumes

and physical volumes are kept the same as those in

the GEANT3 program. Copy numbers of physical

volumes are also kept. It is convenient for the devel-

opers to use this information in the following steps

of simulation like digitization, in a similar way that

they once used in GEANT3. The detector descrip-

tion in the new GEANT4-based simulation program

also keeps exactly the same as the one in the old

GEANT3-based simulation program, which is guar-

anteed by the fully automatic conversion method.

3.2 ROOT geometry for reconstruction

ROOT has been used in PHENIX since its early

version (v2.03). There was no geometry package in

ROOT at that time; most of the functions in geome-

try service of reconstruction were realized by the de-

velopers. Without the support from a detector mod-

eling system, the developers had to use approximate

description for some complex detectors and many ge-

ometry related functions required a lot of work. In

No. 7 YOU Zheng-Yun et alµA method for detector description exchange among ROOT GEANT4 and GEANT3 575

the latest version (v5.14) of ROOT that is used in

PHENIX software, implementation of the new geom-

etry package makes it convenient to develop geometry

service in a more precise and more eﬃcient way.

From the detector description converted from

GEANT3, detector is already constructed in ROOT.

All that developers need to do is to retrieve the infor-

mation from this description and ROOT oﬀers a lot of

useful functions to realize it. For example, boundary

searching is a main function that is used in recon-

struction to judge whether a point is inside one vol-

ume or not. As shown in Fig. 3, for a silicon sensor

sector in the disk of forward vertex sub-detector, the

previous method to judge its boundary was to deﬁne

the range of values on rand φdirections. However,

this is only approximate description since the actual

shape of the sensor is trapezoid. ROOT provides the

function “Contains” to judge any complex shape’s

boundary, which is more precise and is easier to use.

Deﬁning coordinates transformation function is an-

other hard wok for developers, because tree structure

detector description usually creates a lot of levels of

coordinates. ROOT also provides a function to easily

transform the position of a point to any level of co-

ordinate in the detector tree. Another useful ROOT

function is volumes overlap checking, which helps to

ﬁnd the description bugs that may lead to error de-

tector response in PHENIX simulation. By using the

ROOT geometry package, the geometry service de-

cise and more robust.

Fig. 3. Approximate boundary deﬁnition for a

silicon sensor trapezoid.

3.3 Optimization in new sub-detector design

The method of exchanging the detector descrip-

tion between GEANT and ROOT is especially use-

ful in the optimization of the design for a new sub-

detector. The Forward Vertex Silicon detector, as

is shown in Fig.4, is a new sub-detector to be in-

stalled on PHENIX for precise vertex measurement.

The structure of this sub-detector is under design and

changes frequently. There are diﬀerent versions of

geometry of this sub-detector in simulation program.

But it is diﬃcult for the reconstruction geometry ser-

vice to be updated and keep step with the changes all

the time. By converting the GEANT3 detector data

to ROOT, the reconstruction geometry service auto-

matically keeps consistent with the simulation with-

out modifying the source code. The source code only

works on “index” of detector units. All geometry in-

formation of the units, including position, rotation,

shape and material, are retrieved from the detector

description ﬁle using the index. Diﬀerent versions of

detector design can exist at the same time so long as

correct version of detector ﬁle is designated as the in-

put of reconstruction geometry service. This method

makes it easy to optimize the detector design by com-

paring the detector performance after reconstruction.

Fig. 4. The vertex silicon detector design in PHENIX.

4 Summary

In nuclear and particle physics experiments, de-

tector description is used in many applications.

GDML is a very convenient and eﬃcient tool for

describing and managing the geometry of complex de-

tectors. A method to exchange the detector descrip-

tion data among GEANT3, GEANT4 and ROOT is

realized, which reduces by a large amount the work

of the developers. This method provides an easy

way to share the detector description among diﬀer-

ent software programs and helps the optimization of

the design for a new detector.

The authors gratefully acknowledge the collabora-

tion and cooperation of the Oﬄine Software Group

of the PHENIX experiment, especially the PHENIX

Group in Los Alamos National Laboratory for work-

ing on the forward vertex detector upgrade.

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Jul 2003
NUCL INSTRUM METH A

Geant4 is a toolkit for simulating the passage of particles through matter. It includes a complete range of functionality including tracking, geometry, physics models and hits. The physics processes offered cover a comprehensive range, including electromagnetic, hadronic and optical processes, a large set of long-lived particles, materials and elements, over a wide energy range starting, in some cases, from 250 eV and extending in others to the TeV energy range. It has been designed and constructed to expose the physics models utilised, to handle complex geometries, and to enable its easy adaptation for optimal use in different sets of applications. The toolkit is the result of a worldwide collaboration of physicists and software engineers. It has been created exploiting software engineering and object-oriented technology and implemented in the C++ programming language. It has been used in applications in particle physics, nuclear physics, accelerator design, space engineering and medical physics.

Geometry Description Markup Language for Physics Simulation and Analysis Applications

Article

Full-text available

Nov 2006

The Geometry Description Markup Language (GDML) is a specialized XML-based language designed as an application-independent persistent format for describing the geometries of detectors associated with physics measurements. It serves to implement "geometry trees" which correspond to the hierarchy of volumes a detector geometry can be composed of, and to allow to identify the position of individual solids, as well as to describe the materials they are made of. Being pure XML, GDML can be universally used, and in particular it can be considered as the format for interchanging geometries among different applications. In this paper we will present the current status of the development of GDML. After having discussed the contents of the latest GDML schema, which is the basic definition of the format, we will concentrate on the GDML processors. We will present the latest implementation of the GDML "writers" as well as "readers" for either Geant4 , or ROOT ,

ROOT — An object oriented data analysis framework

Article

Apr 1997
NUCL INSTRUM METH A

The ROOT system in an Object Oriented framework for large scale data analysis. ROOT written in C++, contains, among others, an efficient hierarchical OO database, a C++ interpreter, advanced statistical analysis (multi-dimensional histogramming, fitting, minimization, cluster finding algorithms) and visualization tools. The user interacts with ROOT via a graphical user interface, the command line or batch scripts. The command and scripting language is C++ (using the interpreter) and large scripts can be compiled and dynamically linked in. The OO database design has been optimized for parallel access (reading as well as writing) by multiple processes.

The ROOT geometry package

Article

Apr 2003
NUCL INSTRUM METH A

The new ROOT (in: Proceeding of AIHENP’96, Lausanne, September 1996; http://root.cern.ch/) geometry package is a tool designed for building, browsing, tracking and visualizing a detector geometry. The code works standalone with respect to any tracking Monte-Carlo engine; therefore, it does not contain any constraints related to physics. However, the package defines a number of hooks in order to ease-up tracking, allowing user-defined objects to be attached to the basic architectural structures or superimposed on top of them. The modeling functionality was designed to optimize particle transport through complex geometries and it is accessible in an easy and transparent way. The package is currently under development and new features will be implemented, but the basic functionality has already been extensively tested on several detector geometries. The final goal is to be able to use the same geometry for several purposes, such as tracking, reconstruction or visualization, taking advantage of the ROOT features related to bookkeeping, I/O, histograming, browsing and graphical interfaces.

PHENIX detector overview

Article

Mar 2003
NUCL INSTRUM METH A

The PHENIX detector is designed to perform a broad study of A–A, p–A, and p–p collisions to investigate nuclear matter under extreme conditions. A wide variety of probes, sensitive to all timescales, are used to study systematic variations with species and energy as well as to measure the spin structure of the nucleon. Designing for the needs of the heavy-ion and polarized-proton programs has produced a detector with unparalleled capabilities. PHENIX measures electron and muon pairs, photons, and hadrons with excellent energy and momentum resolution. The detector consists of a large number of subsystems that are discussed in other papers in this volume. The overall design parameters of the detector are presented.

Program Library Long Write up W5013

Jan 1993

Program Library Long Write up W5013. CERN, 1993

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81

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F Rademakers

Brun R, Rademakers F. Nucl. Instrum. Methods A, 1997, 389: 81

Jan 2003
469

K Adcox

Adcox K et al. Nucl. Instrum. Methods A, 2003, 499: 469

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Agostinelli S et al. Nucl. Instrum. Methods A, 2003, 506: 250

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Brun R et al. Nucl. Instrum. Methods A, 2003, 502: 676

A method for detector description exchange among ROOT GEANT4 and GEANT3

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